Care patterns in Medicare and their implications for pay for performance.

BACKGROUND Two assumptions underpin the implementation of pay for performance in Medicare: that with the use of claims data, patients can be assigned to a physician or to a practice that will have primary responsibility for their care, and that a meaningful fraction of the care physicians deliver is for patients for whom they have primary responsibility. METHODS We analyzed Medicare claims from 2000 through 2002 for 1.79 million fee-for-service beneficiaries treated by 8604 respondents to the Community Tracking Study Physician Survey in 2000 and 2001. In separate analyses, we assigned each patient to the physician or primary care physician with whom the patient had had the most visits. We determined the number of physicians and practices seen annually, the percentage of care received from the assigned physician or practice, the stability of assignments over time, and the percentage of physicians' Medicare patients who were their assigned patients. RESULTS Beneficiaries saw a median of two primary care physicians and five specialists working in four different practices. A median of 35% of beneficiaries' visits each year were with their assigned physicians; for 33% of beneficiaries, the assigned physician changed from one year to another. On the basis of all visits to any physician, a primary care physician's assigned patients accounted for a median of 39% of the physician's Medicare patients and 62% of Medicare visits. For medical specialists, the respective percentages were 6% and 10%. On the basis of visits to primary care physicians only, 79% of beneficiaries could be assigned to a physician, and a median of 31% of beneficiaries' visits were with that assigned primary care physician. CONCLUSIONS In fee-for-service Medicare, the dispersion of patients' care among multiple physicians will limit the effectiveness of pay-for-performance initiatives that rely on a single retrospective method of assigning responsibility for patient care.